The present invention generally relates to fabrication of semiconductor devices and more particularly to a fabrication method of a semiconductor device using a CMP (chemical mechanical polishing) process and a polishing apparatus used for such a CMP process.
A CMP process is indispensable for fabricating so-called submicron semiconductor devices. Sub-micron semiconductor devices are integrated to form an integrated circuit having a very large scale integration density. In such very large scale integrated circuits, use of a multilayer interconnection structure is essential or inevitable for interconnecting various elements formed on a chip. When forming a multilayer interconnection structure, it becomes necessary to planarize each interlayer insulation film or interconnection layer that constitutes a part of the multilayer interconnection structure.
Conventionally, such a planarization process has been conducted by a CMP (chemical mechanical polishing) process using a colloidal silica slurry or an alumina slurry. A colloidal silica slurry can polish a SiO.sub.2 film efficiently when used together with a liquid oxidant such as H.sub.2 O.sub.2. Further, an alumina slurry is effective for polishing a conductor film such as W. Further, it is known that a ceria (CeO.sub.2) slurry is effective for polishing an oxide film.
However, a ceria slurry is very expensive and the use thereof in a fabrication process of a semiconductor device is not practical. Further, a colloidal silica slurry or an alumina slurry is also expensive, and the use thereof increases the cost of the semiconductor device. There is an estimation that almost one-half of the cost of the CMP process per a wafer is for the cost of the slurry.
Further, it should be noted that the foregoing conventional slurries are used together with a liquid oxidant such as H.sub.2 O.sub.2 in the CMP process as noted already, while the use of such a liquid oxidant tends to cause a problem, particularly when polishing a conductive plug filling a depression or a contact hole, in that a seam, which is formed in such a plug at the time of deposition of a metal layer so as to fill the contact hole, experiences a preferential corrosion as a result of the action of the liquid oxidant. When such a corrosion occurs in a conductive plug, the electrical contact at such a contact hole becomes inevitably unreliable.
Further, in such a conventional CMP process that uses a conventional slurry such as a colloidal silica slurry or an alumina slurry, or alternatively a ceria slurry, there has been a difficulty in removing the slurry from the wafer completely after the CMP process. In particular, the removal of the slurry becomes almost impossible once the slurry is dried up and solidified on the wafer. Because of this, it has been practiced to hold the wafer in water before and after the (CMP process. Even then, a mechanical cleaning process such as the one conducted in a brush scrubber has been indispensable for cleaning the wafer after the CMP process. As will be understood easily, such a mechanical cleaning process tends to cause a damage to the wafer on which semiconductor devices are already formed. In other words, there has been a danger in that the yield of production of the semiconductor device may be reduced as a result of such a mechanical cleaning process.
On the other hand, there has been a proposal to use a slurry containing MnO.sub.2 abrasives for the planarization of a multilayer interconnection structure in the U.S. patent application Ser. No. 08/674,507, now U.S. Pat. No. 5,763,265 and Ser. No. 08/763,051, which are incorporated herein as reference. Under a normal polishing condition, the MnO.sub.2 abrasives act preferentially upon a conductor layer such as W. By using the slurry, therefore, it is possible to stop the polishing exactly and promptly upon the exposure of an oxide film underneath the conductor layer. Further, the proposed slurry has an advantageous feature in that it does not require a liquid oxidant such as H.sub.2 O.sub.2, as the MnO.sub.2 abrasives themselves act as a solid oxidant. Thus, the slurry does not cause the problem of seam corrosion. Further, it was discovered, as in the U.S. patent application Ser. No. 08/674,507, op cit., that the MnO.sub.2 abrasives can be used also for polishing an oxide film, by optimizing the polishing condition.
Further, it was discovered, as in the U.S. patent application Ser. No. 08/763,051, op cit., in that not only the MnO.sub.2 abrasives but also abrasives of Mn.sub.2 O.sub.3 and Mn.sub.3 O.sub.4 are also effective for polishing an insulation film.
When using the slurry containing the abrasives of these various manganese oxides, there is a further advantageous feature in that the cleaning process after the polishing process is substantially simplified, as mentioned in the foregoing U.S. patent application Ser. Nos. 08/674,507 and 08/763,051. More specifically, it is possible to remove the remaining abrasives from the wafer completely after the polishing process, by merely immersing the wafer in an acid bath to which the manganese abrasives are soluble. As MnO.sub.2 is used extensively for the material of a dry cell, the material is mass produced and is readily available. Further, the property of manganese oxide is studied thoroughly in relation to the foregoing extensive industrial use of the material.
It is noted that the manganese oxide abrasives tend to dry up faster than conventional colloidal silica abrasives. However, such a drying up of the abrasives does not cause any serious problem at all, as the abrasives thus adhered to and dried up on a polishing apparatus can be easily dissolved to an acid, provided that the construction of the polishing apparatus is optimized for such an acid cleaning process.
FIGS. 1A and 1B show the construction of a conventional. CMP apparatus 10 designed for use of a conventional slurry that uses conventional abrasives of colloidal silica, alumina or ceria, wherein FIG. 1A shows a side view of the CMP apparatus while FIG. 1B shows a plan view.
Referring to FIGS. 1A and 1B, the CMP apparatus 10 includes an inlet-side wafer carrier 1A for holding unprocessed wafers 11A in a water tank 1 and an outlet-side wafer carrier 1B for holding processed wafers 11B also in the same water tank 1. Further, there is provided a robot 1C in the tank 1 carrying a robot arm 1D, such that the robot 1C takes up a wafer 11A on the wafer carrier 1A by sucking a top surface thereof, wherein the robot 1C forwards the wafer 11A to a load/unload robot 2A that cooperates with a polishing section 2, which forms a part of the polishing apparatus and provided adjacent to the tank 1. Further, the robot 1C takes up a processed wafer from the load/unload robot 2A by sucking a top surface thereof. The robot 1C thereby elevates the robot arm 1D together with the processed wafer and causes the arm 1D to swing as indicated in FIG. 1B. Thereafter, the robot 1C lowers the arm 1D and moves the same along a horizontal shaft 1C' in the direction indicated by arrows.
On the other hand, the foregoing load/unload robot 2A includes arms 2a and 2b movable in up and down directions, and the respective arms 2a and 2b carry trays 2c and 2d for holding a wafer thereon. It should be noted that the arms 2a and 2b are movable to and from the robot 1C in the tank 1 as indicated by an arrow in FIG. 1B and further movable parallel to the drive shaft 1C' of the robot 1C.
Thus, the robot arm 1D is elevated, after picking up an unprocessed wafer 11A from the carrier 1A, to a high position when loading the wafer to the load/unload robot 2A, and the wafer thus held on the arm 1D is placed upon the tray 2c or the tray 2d that is also elevated to a corresponding high position for receiving the wafer 11A. Similarly, a processed wafer on the tray 2c or 2d is picked up by the robot arm 1D when unloading the processed wafer from the load/unload robot 2A.
The CMP apparatus 10 of FIGS. 1A and 1B further includes a platen 3B in the polishing section 2, such that the platen 2 is driven by a motor 3A. Further, the polishing section 2 includes a polishing head 3D driven by another motor 3C, such that the polishing head 3D faces the platen 3B. The head 3D is held on a pillar 3 movably in up and down directions and is loaded with a wafer W from the tray 2c or 2d of the load/unload robot 2A. The head 3D thereby urges the wafer TW thereon against a polishing cloth 3b covering the platen 3B with a predetermined pressure. Thus, the pillar 3 includes a mechanism for causing the head 3D to move up and down.
After polishing, the wafer is returned from the polishing head 3D to the tray 2c or 2d not used in the foregoing loading process. The wafer is then transported to the wafer carrier 1B.
It is possible to operate the CMP apparatus with only one of the arms 2a and 2b. However, use of both arms 2a and 2b increases the efficiency of the polishing operation. As the arms 2a and 2b are movable in the direction parallel to the drive shaft 1C' independently no problem of collision occurs when a wafer on the lower tray 2d is loaded on the head 3D or unloaded thereto from the head 3D.
Further, the CMP apparatus 10 includes a tank 12A for holding a slurry, a slurry feed line 12B connected to the tank 12A for feeding the slurry in the tank, and a drip nozzle 12C provided at an end of the feed line 12B for causing a dripping of the slurry thus supplied thereto on the platen 3B. The slurry thus fed to the platen 3B is then drained from a drain outlet 3E provided underneath the platen 3B.
The conventional CMP apparatus of FIGS. 1A and 1B, however, is designed for a CMP process that uses a conventional slurry such as a colloidal silica slurry or alumina slurry, or alternatively a ceria slurry. Thus, the CMP apparatus lacks the mechanism and capability of acid cleaning process, which should be extremely effective for removing a slurry when applied after the CMP process conducted by a manganese oxide slurry. As explained already, the slurry using a manganese oxide tends to dry up easily as compared with other conventional slurries. Thus, there is a demand for such a CMP apparatus having a feed system for supplying a slurry and further a cleaning system for supplying an acid to the platen on which the polishing is carried out.